This research has two foci. First, the photoreceptor organelles of the crayfish offer a unique set of possibilities for studying a number in interconnected problems in the cell biology of photoreceptors, using a variety of techniques. The dark regeneration of rhodopsin will be studied in relation to i) membrane turnover, ii) retinoid metabolism, and iii) microvillar structure. Isolated rhabdoms will be examined by microspectometry (MSP) and microspectrofluorometry for pigment content, by polarized and electron microscopy for microvillar organization, by autoradiography for protein turnover, and by a variety of biochemical means to study the cytoskeleton. The status of retinoids will be measured by HPLC. Because in the vertebrate eye the mobilization and transport of retinoids during regeneration of rhodopsin is not well understood, it is apropriate to examine any system in which features of these processes may be displayed to advantage. Moreover, the dynamic control of microvillar architecture by light may provide a model in which fundamental features of the microvillar cytoskeleton will come to be understood. As microvilli form the brush borders of epithelial surfaces in the human gut and kidney, this problem is clearly of medical importance. The second focus is the means by which bird retinas are sensitive to short wavelengths, including the near UV. Spectral sensitivity will be measured in opened eyecups by employing selective chromatic adaptation, and the absorption of cone oil droplets and outer segments will be measured by MSP. Understanding these attributes of the avian retina may lead to a better understanding of why some species may be more sensitive than others to short-wavelength damage.